With reference to FIG. 1, frequency modulation two-way radios often use phase locked loop (PLL) frequency synthesizers to meet various transmission and reception needs in the radio, including creation of the modulated signal to be transmitted. Such PLL synthesizers typically include a phase detector (A) and may, depending upon the application, incorporate multiple loop filters (B and C) having different bandpass ranges. These multiple filters can be switched in and out of service as needed. The PLL also includes a voltage controlled oscillator (VCO) (D) that will ordinarily include a tuning varactor (E). Finally, a divide-by-N counter (F) will feedback the output of the VCO (D) to the phase detector (A) in accordance with well understood PLL technique.
Frequency modulation of the VCO (D) output can be achieved in a variety of ways. One common method employs a separate modulation varactor (G) lightly coupled to the VCO tank circuit through a capacitor (H) as shown in FIG. 1. A modulating voltage can be applied to the positive side of the modulation varactor (G) whose capacitance is thereby caused to vary with the modulating voltage, thus causing the resonant frequency of the VCO tank and the frequency of oscillation of the VCO (D) to vary with the modulating voltage. (Those skilled in the art of synthesizer modulation will realize that modulating the VCO alone will give a flat frequency response only for modulating frequencies above the bandwidth of the loop since the negative feedback action of the loop will tend to cancel the modulation at frequencies below the loop bandwidth. This invention does not address this aspect of frequency synthesizer modulation; it does, however, provide for obtaining flat frequency response for the VCO modulation port if operated in an open-loop mode, thus allowing truly flat response in the closed-loop mode when used in conjunction with means for obtaining flat closed-loop response, such as simultaneous modulation of the VCO and reference frequency.)
Prior art devices also often require a means of shifting the tuning range of the VCO (D). For example, a 20 Mhz band of frequencies might be split into two 10 Mhz ranges in order to keep the VCO steering line sensitivity lower and to allow lighter coupling of the main tuning varactor to the VCO tank in order to minimize the noise degradation caused by the typically low Q of the varactor. In the prior art shown in FIG. 1, range shifting is accomplished by switching in or out of the VCO tank an auxiliary inductance or capacitance (L/C) by means of a switching device (J) with suitable RF characteristics, such as a PIN diode.
There are problems associated with the above prior art techniques. One or more additional varactors and associated circuitry are required to accommodate the modulation technique. The PIN diode switching technique employed for range shifting requires a significant current draw increase in order to turn the diode "on", and also can cause a degradation of the VCO sideband noise due to the on-resistance of the PIN diode which degrades the Q of the VCO tank.
Accordingly, a need exists for a range shifting VCO in a PLL synthesizer that can provide a modulated output without requiring undue current draw, without undue noise degradation, and without requiring undue additional complexity and parts.